There are various conditions that may cause an arc fault. Corroded, worn or aged wiring or insulation, worn power cords, old wall outlets with insufficient contact pressure, electrical stress from repeated overloading, etc., may result in an arc fault. These conditions may damage the insulation of the wiring and create excessive heating temperatures. Arc faults may result in a fire depending on various conditions, such as if combustible materials are in close proximity.
There are also various conditions that may result in a false arc fault. For example, the occurrence of an arc fault in one branch circuit of a power distribution system may generate a false arc in another branch circuit. As a result, circuit breakers in more than one branch circuit may erroneously trip. Another example is a relatively noisy load such as an electric drill creating a high frequency disturbance in the circuit, which may appear to be an arc fault.
There are two types of arc faults that may occur in a home. A first type is a high-energy arc that may be related to high current faults; a second type is a low current arc that may be related to persistent momentary contact of electrical conductors. The first type may result from inadvertent connection between a line conductor and neutral conductor or a line conductor and ground. The first type may draw current that is above the rated capacity of the circuit, arcing as the contacts are physically joined.
The other type of arc fault, the momentary contact of electrical conductors, may be considered more problematic. Since the current in the arc may be limited to less than the trip rating of an associated circuit breaker, such arcs may become persistent without observation and may result in certain conditions. Contact arcs may be caused by springs in switches that become worn which, in turn, may reduce the forces that hold electrical contacts together. As the electrical contacts heat and cool down, the conductors may touch and separate repeatedly, thereby possibly creating arcs known as "sputtering arcs."
Contact arcs or sputtering arcs may also be observed in contacts which are made from different materials. For example, aluminum wiring which contacts copper wiring may oxidize at the contact points. In this case a non-conductive layer may build up over time between the contact points and arcing may result. Sputtering arcs may also be observed in extension cords having insufficient current carrying capacity. As the plug is heated by resistance heating, insulating materials around the contacts may eventually melt and flow into the contact area, preventing proper contact from being made. The current in the conductors may produce magnetic repulsion forces which may push the conductors apart, resulting in an arc. The arc may be extinguished as the current passes through zero. Mechanical or electro-static forces may bring the conductors back into contact, and the cycle may be repeated.
It is believed that various circuit breakers are not specifically designed to guard against sputtering arcs. Special purpose detectors have been designed to detect sputtering arcs and, when detected, trip the circuit breakers. Some detectors are believed to depend on the sputtering arcs exceeding a predetermined current or voltage threshold before tripping the circuit breaker; other detectors are believed to depend on the sputtering arcs having a specific high frequency signature. Still other detectors are believed to depend on the sputtering arcs producing a wideband high frequency noise ranging from 10 kHz to 1 GHz while the arc is conducting current. These detectors may require that no noise be produced while the arc is not conducting current, i.e., during the gaps between arc conduction. These detectors use various processing techniques to analyze the repetitive patterns in the noise.
It is believed that a need still exists for a sputtering arc fault detector that does not depend on the frequency content of the noise generated by the arc or on relatively involved analysis techniques to determine the arc signature characteristics.